44 COLOUR IN NATURE chap. 



In Crustacea there is probably only one series of 

 blue colouring- matters, the so-called soluble blues, 

 which will be more fully described in the chapter on 

 the group. 



Among Mollusca we have notably the blue pig- 

 ment of lanthina and the purple one of Aplysia, the 

 undescribed colouring matters of the shells of mussels, 

 of the mantle of Tridacna, and of others. 



Among Vertebrates blue pigments do not appear 

 to have been as yet described, except in the case of 

 the blue pigments of the eggs of many birds. These 

 pigments are apparently derivatives of haemoglobin. 



A green colour is of course almost universally 

 distributed among plants, where it is due to the 

 important pigment chlorophyll. Among animals 

 green like blue is rare as a pigmental colour, except 

 in simple organisms. The questions connected with 

 the green colours of animals have been greatly com- 

 plicated by the common habit of hastily assuming 

 that any pure green colour in animals is likely to be 

 due to chlorophyll. Chlorophyll occurs apparently 

 in many Protozoa, in Hydra viridis, in the fresh-water 

 sponge Spongilla, and in some Turbellarian worms. 

 Its occurrence in numerous other animals, especially 

 worms and insects, has been often asserted, probably 

 in some cases because the gut contained unaltered 

 chlorophyll which gave the characteristic spectrum. 

 " Modified chlorophyll " is said by Mr. Poulton to 

 colour the tissues of many caterpillars. A group of 

 pigments which resemble chlorophyll, and which 

 have been often mistaken for it, are apparently 

 widely spread among Invertebrates. They usually 

 occur in connection with internal organs, and are 



